The following publications are representative of the most relevant prior art known to Applicants at the time of filing of this application:
______________________________________ U.S. PAT. NOS. ______________________________________ 3,959,557 May 25, 1976 A. R. Berry 4,134,759 January 16, 1979 S. Yajima et al. 4,252,062 February 24, 1981 N. Nishida 4,505,720 March 19, 1985 T. Gabor et al. November 24, 1987 D. J. Emmanuelli ______________________________________